In recent years residential house construction has been altered to make them more energy efficient and to reduce heating costs. One method used to achieve this has been to seal the structure to reduce the amount of cold outside air infiltrating into the living space. From an energy perspective this is a good approach but from occupancy perspective there are potential problems. People within the house require fresh air to breath, and fresh air also removes toxins and odours that can accumulate within the house. To deal with these conflicting needs for fresh air the National and Provincial Building Codes have established minimum ventilation standards for residential dwells. Typical standards require 0.3 air changes per hour for the dwelling (either year round or only during the heating season).
Inherently, a lot of house air goes up the chimney from the combustion chamber and must be replaced by outside air. In modern houses have become increasingly air tight in order to conserve energy, particularly in colder climates. This has led to a need for ensuring adequate replacement of air in buildings where there is a combustion heating system, such as oil or gas. It is known to provide a duct from the outside emptying into the building basement to provide such make-up air. Typically, an inlet duct is provided to deliver outside air to the vicinity of the combustion chamber for provision of such makeup air. This approach may create some problems for both the building occupants and the heating system. A better idea is to introduce the make-up air into the cold air return duct of the furnace, where it is mixed with air that is going to be heated on the heating coils of the furnace and distributed to the house through the hot air plenum.
Practically all houses and small commercial buildings have a tendency toward a negative internal pressure due to forced exhausting of internal air. This is due mainly to expelling undesirable air from a building by using an exhaust fan blowing out and passively supplying replacement fresh air via a vent.
An improvement over this is to have an outside air duct leading into the cold air return on the furnace, where it mixes with cold air returning from parts of the house, and is then fed to the heat exchanger from which it proceeds to the hot air plenum, providing heated air through the building. For example, as is disclosed by Blotham et al. in Canadian Patent No. 685,597, issued May 5, 1964.
Hence, the idea of introducing outside air into the return air side of the furnace is well known. However, the increased ventilation requirements, resulting from increased air tightness of modern house, has increased the requirement for fresh outside air. For example, Sheperd et al. in U.S. Pat. No. 4,730,771, issued Mar. 15, 1988, disclose a hot air furnace in which hot air from the hot air plenum is fed into the make-up air duct, and then fed into the return air plenum of the furnace. The hot air is used to draw the make-up air. A damper within the make-up air duct at the junction of the hot air supply regulates air flow.
Many proposals introduce a heat exchanger into the chimney flue, for example U.S. Pat. No. 2,962,218 issued to F. Dibert, Nov. 29, 1960. The introduction of heat exchangers into the chimney flue may cause problems. For example, when this fresh air crosses the heat exchanger, under certain circumstances, a rain forest condition may be created in the heat exchange chamber. Additionally, the heat exchanger may not adequately handle an extreme temperature gradient between flue gases and incoming outside air. Further, the flue gases may contain toxic mist. Consequently, the life expectancy of heat exchangers and flues may be very short. It has been determined experimentally that the tempering the air with circulation air improves the temperature gradient across the heat exchanger.
When the outside temperature drops to the range of -22.degree. to -40.degree. F. (-30.degree. to -40.degree. C.), ensuring a regulated supply of the outside air is critical. If there is insufficient air, the combustion in the furnace is incomplete and the supply of fresh air for the occupants becomes seriously limited.
Building codes are beginning to require that any incoming air be warmed to a minimum 55.degree. F.(13.degree. C.) before it is introduced into the premises. Major problems arise from the need to heat up the outside air before it is fed into any plenum. As discussed above, flue gas heat exchangers have been proposed. The use of electrical heating coils for this purpose has been suggested, but clearly this is not the best solution, as it introduces an electrical heating element into the combustion heating system of the house.
Canadian Patent Application 2,084,753 discloses a mixing device wherein fresh air is induced through a nozzle of an adjustable aperture. In one embodiment, the fresh air is mixed with heated air. This arrangement may require too large of a air volume through the nozzle to be practical to provide desired fresh air induction rates.